Process for monitoring ion-assisted processing procedures on wafers and an apparatus for carrying out the same

ABSTRACT

Disclosed is a process and an apparatus for monitoring ion-assisted processing procedures on wafers in a process chamber. 
     In accordance with the present invention the known process, respectively the known apparatus is improved by, for the determination of the energy of the ions and/or of the divergence of the ion beam, the red shift and/or the blue shift of emission lines resulting from the reflection of particles an the surface of the wafer being determined from the gases present in the process chamber.

This is a continuation of application Ser. No. 07/768,436, filed Nov. 7,1991, now U.S. Pat. No. 5,319,197.

FIELD OF THE INVENTION

The present invention relates to a process for monitoring ion-assistedprocessing procedures on wafers in a process chamber and an apparatusfor carrying out the same.

BACKGROUND OF THE INVENTION

ion-assisted processing procedures are employed both for the removal ofmaterial as well as for the application of material, respectively thecoating of semiconductor, metal, glass or plastic substrates. Theaforementioned materials are summarily referred to hereinafter as"wafer".

Processing procedures in which materials are removed may be, by way ofillustration, reactive ion etching "RIE", magnetically enhanced RIE"MERIE", triode etching, reactive ion beam etching "RIBE", chemicallyassisted reactive ion beam etching "CAIBE", ion milling or sputtering.

In all ion-assisted processing procedures it is essential for conductingthe processing to know the energy of the ions and the divergence of theion beam. Furthermore, in a number of cases it is of importance todetermine the possible charging of the wafer as in particular, with thininsulating layers even minimal charging can cause voltage breakdowns.Furthermore, by way of illustration, in the case of RIE it is necessaryto determine the etching rate "in situ", respectively to have an"endpoint control" of the processing procedure.

Determination of the aforementioned values, respectively processingparameters, is only possible in accordance with the state of the art bymeans of a number of different measuring processes, which, undercircumstances, have to be employed simultaneously in a process chamber.Thus presently the divergence of the ion beam is usually measured(measurement of current) with "conductive cups". The arrangement of themeasuring devices in a process chamber necessary for this purposehowever "disturbs" the ion beam and therefore the processing procedure.The thickness of the layer, on the other hand, is usually measuredinterferometrically so that the process chamber has to be designed insuch a manner that, by way of illustration, a laser interferometer maybe utilized, cf. e.g. the PCT application WO 88/07261. The use of anoptical spectrometer for the chemical identification of reactionproducts of the surface of a wafer is also described in this printedpublication.

SUMMARY OF THE INVENTION

The present invention provides a process for monitoring ion-assistedprocessing procedures on wafers in a process chamber with whichmagnitudes, i.e., processing parameters, essential for conducting theprocessing may be determined practically simultaneously with a minimalamount of time, effort and cost.

In the invented process it was recognized that the magnitudes, i.e.process parameters, of importance for the ion-assisted processingprocedures on wafers can be determined by means of emission spectographyof the operational gases present in the process chamber. Operationalgases mean all the gases present in the process chamber, thus, by way ofillustration, the ions i.e. fast neutral particles) supplied by the ionsource, the background gas present in the chamber (typical pressure inthe operation chamber ≈10⁻⁶ ₁₄ 10 ⁻⁷ mbar), additional operational gaspresent in the process chamber as is the case in CAIBE processes orcoating, or reaction products of the surface.

On the way between the ion source and the wafer surface, a part of theions collides with neutral particles in the process chamber. With acertain degree of probability, these collision processes result inelectronic excitation of the colliding ions and particles. The lightgenerated in "neutralizing" the electronic excitation is "Dopplershifted" toward longer wavelengths, that is "red-shifted", compared tothermal particles due to the velocity of the colliding particles andions. From the distance between, e.g., the red-shifted maximum and theunshifted maximum, which occurs with thermal particles, therefore thevelocity and thus the energy of the particles can be calculated.

Moreover, with a certain degree of probability, the particles reflectedby the surface of the wafer are electronically excited by the collisionwith the wafer. In the case of these particles, a blue shift of theemission lines occurs.

Also from the distance between the blue-shifted maximum and theunshifted maximum, the particle energy can again be determined.

Furthermore, the spectral width of the distribution of the emissionsabout the red-shifted or the blue-shifted maximum permits drawingconclusions about the beam divergence

Moreover, the charging of the wafer results in counter voltage whichslows down the ions thus reducing the spectral red and blue shift. Forthis reason, in accordance with the invention in order to determinepossible charging of the wafer, the spectral position of the red-shiftedand/or blue-shifted maximum, i.e. the temporal change of the spectralposition, is ascertained.

This permits, i.a., for the first time a reliable DC bias measurement,i.e. a determination of the potential difference between plasma andwafer in RIE processing procedures. In cathodic extraction (DC, HF orNF) via a plasma peripheral layer, as is the case in RIE processes andin RI deposition processes, not only is determination of theautomatically occuring potential difference possible via the energy ofthe particles, but, in particular, in the case of low pressure also thedetermination of the distribution of the energy of the particles.

A part of the emitted light enters the detection device directly,whereas the other part of the light which entered the detection devicewas first reflected at the surface of the wafer.

If the wafer is a coated wafer, by way of illustration a Si-wafer coatedwith a SiO₂ layer, there are interferences of the light reflected at theupper and Lower boundary surfaces. Thus the intensity of the receivedlight is dependent on the thickness of the layer.

For this reason, in accordance with the invention, in order to determinethe current thickness of the processed layer, the temporal change of theintensity of the blue-shifted or the unshifted spectral lines and/or thespectral position of the blue-shifted maximum is ascertained.

Both the values of the red as well as the blue shift of the emissionlines of the operation gases in the process chamber can, by way ofillustration, be measured with a monochromator having a photomultipliertube connected thereafter or having other detection devices, such asinterference filters. In any case, it is preferable if the detectiondevice ascertains the various emissions collinearly, i.e. in the axis ofthe ion beam (claim 6). This has both the advantage that in that casethe magnitude of the measured shift is the greatest with given particlevelocity and also the advantage that the determination of the thicknessof the layer is simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is made more apparent in the following sectionusing preferred embodiments with reference to the accompanying drawings.

FIG. 1 shows the principle construction of an apparatus for carrying outthe invented process;

FIGS. 2 shows measuring results obtained with the apparatus illustratedin FIG. 1;

FIG. 3 shows measuring results obtained with the apparatus illustratedin FIG. 1;

FIG. 4 shows measuring results obtained with the apparatus illustratedin FIG. 1;

FIG. 5 shows measuring results obtained with the apparatus illustratedin FIG. 1.

DESCRIPTION OF THE DRAWINGS

The apparatus depicted in FIG. 1 has an ion source 1 which supplies ionsas plasma from which a number of grating 2 accelerates an ion beam 3 ona not depicted wafer. In the illustrated preferred embodiment, atwo-grating optics is utilized. It is expressly pointed out that thedesign of the ion source for the invented process is not decisive andthat the invented process, by way of illustration, may also be appliedif --as, by way of illustration, in the case of the RIE process --ionsare accelerated out of a plasma onto walls, etc., via an automaticallyoccuring potential so that in a real sense it actually is not an ionbeam.

The ions of the ion beam collide with the other particles in proportionto the gas background pressure in the process chamber (approx. 10⁻⁷-10⁻⁶ mbar). This is symbolized in FIG. 1 by the reference number 4,with gas, i.e. operation gas, referring to all the gases present in theprocess chamber, thus, by way of illustration, the ions (respectivelyrecombined neutral particles) supplied by the ion source, the backgroundgas present in the chamber, additional operation gas present in theprocess chamber as is the case in CAISE processes, or reaction productsfrom the surface.

These collision processes result with a certain degree of probability inelectronic excitation of the colliding ions and particles. The lightgenerated in "neutralizing" the electronic excitation is detected in thepreferred embodiment illustrated in FIG. 1 with a monochromator 5 whichhas a photomultiplier 6. The axis 5' of the monochromator 5 is arrangedin the illustrated preferred embodiment under an angle Γ=30° to the axisof the ion beam 3. Following collision, the direction of motion of the"colliding ion" includes the angle α with the axis of the ion beam.v.sub.α is the velocity of the ion following collision.

In the following measuring results obtained with the apparatusillustrated in FIG. 1 are explained using FIGS. 2 to 5, withmeasurements in the range of the unshifted (neutral) argon lineλ0=696.543 nm (6965.43 Å) being illustrated. Naturally, depending on theinstance of application practically any emission lines of the utilizedions and/or particles present in the process chamber can be evaluated.

FIG. 2 depicts a typical measurement in the range of Ar I line if nosubstrate is present in the process chamber. In addition to the "sharp"unshifted emission line n stemming from thermal particles, a smaller"red-shifted" maximum R stemming from the fast particles is observed.

In this case the wavelength shift Δλ, d.h. the spectral distance betweenthe unshifted maximum n and the red-shifted maximum R, is given by theprojection of the particle velocity v.sub.α on the axis of themonochromator axis 5':

    Δλ=λ.sub.0 *V.sub.α * cos (Γ±α)

λ0 is the wavelength of the unshifted emission line). As the deflectionangle α, among other things is a function of the beam divergence, theparticle velocity can be determined from the distance of the red-shiftedpeak maximum from the unshifted maximum and therewith the energy of theion beam and from the width of the red-shifted maximum at leastqualitative conclusions can be drawn about the divergence of the ionbeam.

FIG. 3 shows the influence of various beam divergences on the "shape" ofthe red-shifted maximum, with the measuring result with a beam havingcomparatively large divergences being depicted in section (a) and insection (b) the result with a beam having a small divergence. The beamdata are to be drawn from the respective section. U_(BEAM) designatesthe velocity voltage and U_(acc) the voltage applied to the secondgrating of the two-grating optics (related to 0 V).

FIG. 4 depicts the result obtained if a wafer is disposed in the ionbeam 3. Due to reflection at the surface of the wafer, a maximum Bshifted to shorter wavelengths (blue-shifted) also occurs, from thedistance of which to the unshifted maximum the particle energy can bedetermined and from the width of which information can be gained aboutthe beam divergence.

FIG. 5 depicts the temporal variation of the intensity of the"blue-shifted" maximum in the case of a coated wafer. Due to the shortlifetime (approx. 10-100 ns) of the electronically excited states, onlyparticles are detected in the emission spectrum which are in a rangehaving a thickness of several millimeters above the surface of thewafer. A part of the emitted light enters directly into the detectiondevice, whereas the other part of the light which entered directly intothe detection device was first reflected at the surface of the wafer.

If the wafer is a coated wafer, by way of illustration, a Si-wafercoated with a layer of SiO₂, there are interferences of the lightreflected at the upper and the lower boundary surfaces. Thus theintensity of the received light is dependent on the thickness of thelayer.

The "sputtering" processing procedure illustrated, by way of example, inFIG. 5 yields a purely sinus-shaped interference signal, from the"period" of which the etching rate R_(a) can be determined using thefollowing formula:

    R.sub.a =λ/2*t√(n.sup.2 -sin.sup.2 Γ).

with t being the temporal interval between two maxima (minima) and n therefractive index of the upper layer.

If the oxide layer is removed in the case of the illustrated embodiment,the signal no longer changes so than a reliable end point control isyielded.

In the illustrated preferred embodiment the axis 5' of the monochromator5 and the axis of the ion beam 3 include an angle Γ<≦0°. In manyapplications, however, it is preferred if the angle is Γ=0°, as in thatevent the determination of the thickness of the layer is simplified andthe shift of the maxima is the greatest with the given particlevelocity. This can be realized by utilizing a "transparent" source 1,"after" which the detection device for the emitted light is disposed. Aparticular advantage of the use of shifted lines is namely that in thisway it is possible no "look through" the ion source and the extractionoptics.

Furthermore, in order to determine the etching rate, it is also possibleto evaluate the unshifted emission line. A spectral separation of thelines is not necessary for the evaluation. Other devices may also beemployed as the detection device, such as interference filters withphoto-multipliers connected thereafter or other light receivers.

In any event, the invented process, however, has the advantage that itis possible to practically simultaneously determine the energy of theions, the beam divergence, the thickness of the layer and the end pointof the processing as well as the charging of the wafer.

The present invention may be utilized for monitoring all ion-assistedprocessing procedures and, in particular, in the case of removal ofmaterial and application of material, respectively coatingsemiconductor, metal, glass or plastic substrates.

What is claimed is:
 1. Process for monitoring a thickness of a thin filmon an underlying substrate in a process chamber, the process comprisingthe steps of:using as a source of illumination an optical emission of alight emitting medium which is used for processing or is generatedduring processing; and reflecting said optical emission by one of saidthin film and said underlying layer, said reflecting optical emissioninterfering such that a detected light is a periodic function of saidthickness of said thin film.
 2. Process according to claim 1, whereinsaid light emitting medium is one of a plasma and an ion beam. 3.Process according to claim 1, wherein a change in thicknesscorresponding to one cycle or period of reflected light intensitydivided by the duration of that cycle is proportional to an etching ordeposition rate.
 4. Process according to claim 1, wherein the wavelengthof the detected light can be selected.
 5. Process according to claim 1,wherein an etching or deposition rate is determined in situ during saidprocess.
 6. Process according to claim 2, wherein an etching ordeposition rate is determined in situ during said process.
 7. Processaccording to claim 3, wherein an etching or deposition rate isdetermined in situ during said process.
 8. Process according to claim 4,wherein an etching or deposition rate is determined in situ during saidprocess.
 9. Apparatus for carrying out the process according to claim 1,wherein the source of illumination is a light emitting medium which isused for processing or is generated during processing, the light istransmitted through means for monochromating the light to a detector todetect the intensity of the light.
 10. Apparatus according to claim 9,wherein the light emitting medium is one of a plasma and an ion beam.11. Apparatus according to claim 9, wherein said means formonochromating is at least one of a fixed-wavelength filter and atunable-wavelength filter.
 12. Apparatus according to claim 9, wherein adetector is one of a charged coupled device (CCD), an array ofphotodiodes, and a photomultiplier.